ALBERT

Description: Exceptional outcrops recently exposed in the Koniambo massif allow the study of the serpentine sole of the peridotite nappe of New Caledonia (southwest Pacific Ocean). Many magnesite veins are observed, with characteristics indicating that they were emplaced during pervasive top-to-the-southwest shear deformation. The oxygen isotope composition of magnesite is homogeneous (27.4 〈 18 O 〈 29.7), while its carbon isotope composition varies widely ( – 16.7 〈 13 C 〈 – 8.5). These new data document an origin of magnesite from meteoric fluids. Laterization on top of the peridotite nappe and carbonation along the sole appear to represent complementary records of meteoric water infiltration. Based on the syn-kinematic character of magnesite veins, we propose that syn-laterization tectonic activity has enhanced water infiltration, favoring the exportation of leached elements like Mg, which has led to widespread carbonation along the serpentine sole. This calls for renewed examination of other magnesite-bearing ophiolites worldwide in order to establish whether active tectonics is commonly a major agent for carbonation.

Description: We perform a detailed modelling of the post-outburst surface emission of the low magnetic field magnetar SGR 0418+5729. The dipolar magnetic field of this source, $B=6\,{\times\, 10^{12}}{\,{\rm G}}$ estimated from its spin-down rate, is in the observed range of magnetic fields for normal pulsars. The source is further characterized by a high pulse fraction and a single-peak profile. Using synthetic temperature distribution profiles, and fully accounting for the general-relativistic effects of light deflection and gravitational redshift, we generate synthetic X-ray spectra and pulse profiles that we fit to the observations. We find that asymmetric and symmetric surface temperature distributions can reproduce equally well the observed pulse profiles and spectra of SGR 0418. None the less, the modelling allows us to place constraints on the system geometry (i.e. the angles and that the rotation axis makes with the line of sight and the dipolar axis, respectively), as well as on the spot size and temperature contrast on the neutron star surface. After performing an analysis iterating between the pulse profile and spectra, as done in similar previous works, we further employed, for the first time in this context, a Markov-Chain Monte Carlo approach to extract constraints on the model parameters from the pulse profiles and spectra, simultaneously. We find that, to reproduce the observed spectrum and flux modulation: (a) the angles must be restricted to 65° + 125° or 235° + 295°; (b) the temperature contrast between the poles and the equator must be at least a factor of ~6, and (c) the size of the hottest region ranges between 0.2 and 0.7 km (including uncertainties on the source distance). Lastly, we interpret our findings within the context of internal and external heating models.

Description: : The provenance of middle Eocene to early Miocene sedimentary rocks cropping out in the Sulaiman fold and thrust belt has been determined examining the mineralogy, bulk-rock major and trace elements, and Nd–Sr isotopes. The older (50–30 Ma) deposits are characterized by a mixed orogenic provenance with a major contribution from the Karakorum and the Tethyan belt ( c . 80%). As the 50–30 Ma deposits have a provenance distinct from that of coeval Subathu, Khojak and Ghazij shallow marine formations of India and Pakistan, we propose that they were deposited as a distinct delta system that once fed the Palaeo-Indus fan. We document a major change in provenance that occurred before the early–late Oligocene transition at c . 30 Ma. This change in provenance is marked by the appearance of chlorite and monazite and a shift toward more radiogenic Nd–Sr isotopic compositions. We interpret this change as the result of the exhumation and erosion of the proto-Higher Himalaya. The 30–15 Ma sampled rocks are characterized by a major contribution from the Tethyan belt and the Higher Himalayan Crystallines (70–90%) and a subordinate contribution (10–30%) from the Karakorum, Ophiolitic Suture and Trans-Himalaya. As the Nd(0) values of our 30–15 Ma samples are similar to those of the Palaeo-Indus fan deposits, we suggest that the 30–15 Ma sedimentary rocks of the Sulaiman fold and thrust belt were the fluvial onshore record of the Indus fan. Other coeval deposits of India and Pakistan recorded similar increasing exhumation of the Higher Himalaya range, so that we postulate that these sedimentary rocks all derived from the Palaeo-Indus drainage basin. This would suggest that the modern Indus drainage basin is no younger than 30 Ma.

Description: Fast uplift and exhumation of the Himalaya and Tibet and fast subsidence in the foreland basin portray the primary Neogene evolution of the Indian-Eurasian collision zone. We relate these events to the relative northward drift of India over its own slab. Our mantle-flow model derived from seismic tomography shows that dynamic topography over the southward-folded Indian slab explains the modern location of the foreland depocenter. Back in time, our model suggests that the stretched Indian slab detached from the Indian plate during the indentation of the Eurasian plate, and remained stationary underneath the northward-drifting Indian continent. We model the associated southward migration of the dynamic deflection of the topography and show that subsidence has amounted to ~6000 m in the foreland basin since 15 Ma, while the dynamic surface uplift of the Himalaya amounted to ~1000 m during the early Miocene. While competing with other processes, transient dynamic topography may thus explain, to a large extent, both the uplift history of the Himalaya and subsidence of its foreland basin, and should not be ignored.

Description: 〈span〉〈div〉Abstract〈/div〉Fluorine (50–650 ppm), bromine (0.03–0.3 ppm), iodine (0.03–0.4 ppm), boron (20–100 ppm) and nitrogen (5–45 ppm) concentrations are elevated in antigorite-serpentinites associated with the Tso Morari ultrahigh-pressure unit (Himalayas) exhumed from 〉100 km depth in the mantle wedge. These fluid-mobile elements are likely released with fluids from subducted marine sediments on the Indian continental margin to hydrate overlying forearc serpentinites at shallow depths. Of these, F and B appear to remain in serpentinites during the lizardite-antigorite transition. Our results demonstrate serpentinites as transient reservoirs of halogens, B, and N to at least 100 km depth in the mantle wedge, and likely deeper in colder slabs, providing a mechanism for their transport to the deeper mantle.〈/span〉

Description: How and when the Tibetan plateau developed has long been a puzzling question with implications for the current understanding of the behaviour of the continental lithosphere in convergent zones. We present and discuss recent data acquired in geology and geophysics and through igneous and metamorphic petrology and palaeo-altitude estimates. It appears from this research that Tibet initially resulted from the accretion of the Gondwana continental blocks to the southern Asian margin during the Palaeozoic and Mesozoic eras. These successive accretions have potentially favoured the creation of local landforms, particularly in southern Tibet, but no evidence exists in favour of the existence of a proto-Tibetan plateau prior to the Cenozoic. Moreover, before the India-Asia collision, the Tibetan crust had to be sufficiently cold and rigid to transfer the horizontal forces from India to northern Tibet and localize the deformation along the major strike-slip faults. However, these successive accretions associated with subductions have metasomatized the Tibetan lithospheric mantle and largely explain the potassium- and sodium-rich Cenozoic magmatism. Another consequence of this contamination by fluids is the softening of the Tibetan lithosphere, which favoured intra-continental subductions. The timing and the geochemical signatures of the magmatism and the palaeo-altitudes suggest the early growth of the Tibetan plateau. By the Eocene, the southern plateau and the northern portion of Himalaya would be at an altitude of approximately 4000 meters, while the central and northern Tibetan plateau was at altitudes of approximately 2000 to 3000 meters at the Eocene-Oligocene transition. From all of these data, we propose a model of the formation of the Tibetan plateau coupled with the formation of Himalaya, which accounts for more than 2500 km of convergence accommodated by the deformation of the continental lithospheres. During the early Eocene (55-45 Ma), the continental subduction of the high-strength Indian continental lithosphere dominates, ending with the detachment of the Indian slab. Between 45 and 35 Ma, the continental collision is established, resulting in the thickening of the internal Himalayan region and southern Tibet and the initiation of intra-tibetan subductions. By 35 Ma, the southward subduction of the intra-tibetan Songpan-Ganze terrane ends in slab break-off and is relayed by the oblique subduction of the Tarim the Athyn Tagh propagated northeastward beneath the Qilina Shan. Southward, the dextral Red River fault accommodated the southeastward extrusion of the Indochina block. During the Miocene, specifically, between 25 and 15 Ma, the Indian slab undergoes a second break-off, while the central part of Tibet is extruded eastward. Northward, the continental subduction beneath the Qilian Shan continues. Discontinuous periods of magmatic activity associated with slab detachments play a fundamental role in the convergence process. These periods lead locally to a softening of the mid-crust by magma heat transfer and to the granulitisation of the lower crust, which becomes more resistant. We propose that due to these alternating periods of softening and hardening of the Tibetan crust, the rheological behaviour of the convergence system evolves in space and time, promoting homogeneous thickening periods alternating with periods of localised crustal or lithospheric deformations.